At the time of producing various components, such as components for automobiles or components for construction machines, a process is performed which involves: subjecting a hot-rolled material of carbon steel, alloy steel or the like to spheroidizing treatment to give cold workability thereto; cold-working the material; subjecting the material to cutting or some other working to be formed into a predetermined shape; and then subjecting the material to quenching and tempering to adjust the final strength of the material.
In recent years, the shape of components has tended to be made complicated and large. With the tendency, steel material has been required to be made still softer in a cold working step, thereby preventing the steel material from being cracked and improving the lifespan of the mold (concerned). In order to be made still softer, the steel material is subjected to spheroidizing treatment for a longer period. However, to make the period for the thermal treatment too long causes a problem from the viewpoint of energy saving.
Hitherto, several methods have been suggested for obtaining a softness equivalent to that of ordinary spheroidized material even when the period for spheroidizing is made short or the spheroidizing period is omitted. As such a technique, Patent Literature 1 discloses a technique of specifying pro-eutectoid ferrite- and pearlite-microstructures, adjusting the average grain diameter thereof into the range of 6 to 15 μm, and further specifying the volume proportion of ferrite, thereby making a rapid attainment of spheroidizing treatment compatible with the cold forgeability of the steel. When the microstructure is made fine, the spheroidizing treatment period can be shortened; however, when a material is subjected to an ordinary spheroidizing treatment (annealing treatment for about 10 to 30 hours), the material is insufficiently softened.
Patent Literature 2 discloses a technique of specifying not only the volume proportion of pro-eutectoid ferrite but also the respective volume proportions of pearlite-microstructure and bainite-microstructure, thereby making it possible to shorten the period for annealing. According to such a technique, the steel attains a rapid spheroidization; however, the steel is not yet sufficiently softened. Additionally, the steel is made into a mixed microstructure of bainite and pearlite, so that it is feared that the steel becomes uneven in hardness after being spheroidized.